The present invention relates generally to adjustable speed drive (ASD) circuits and, more particularly, to a system and method for minimizing and interrupting capacitor fault energy in ASD circuits to protect the electrolytic DC link capacitors from being damaged by a fault condition.
One type of system commonly used in industry that performs power conversion is an ASD circuit, also known as a variable frequency drive (VFD) circuit. An ASD is an industrial control device that provides for variable frequency, variable voltage operation of a driven system, such as an AC induction motor. A typical ASD circuit 10 is illustrated in FIG. 1. ASD or VFD circuit 10 includes an electromagnetic interference (EMI) filter 12 provided between a drive input 14 and input rectifier terminals 16. The terminals are connected to a rectifier bridge 18 that converts an AC input power provided at drive input 14 into a DC power, with the rectifier bridge 18 being in the form of an “uncontrolled” rectifier bridge that includes an arrangement of six diodes 20—with a pair of diodes 20 arranged in series on each of the three input phases. The ASD circuit 10 additionally includes a DC link 22 that receives the DC power from rectifier bridge 18; a DC link capacitor bank 24 having two capacitors 26, 28 across DC link 22; two inductors 30 coupled in series with and on either side of the rectifier bridge 18 on DC link 22; a pre-charge circuit 32 to control and limit inrush current into DC link capacitor bank 24 during power up; a brake chopper 34 to discharge the DC link capacitor bank 24; an inverter 36 to convert the DC power to AC power; and an output 38 coupled to the inverter 36.
While the above described architecture of the ASD circuit 10 allows for efficient operation thereof, the ASD circuit lacks sufficient protection against various internal and external fault conditions that could damage ASD circuit. One internal fault condition that occurs in ASD circuits is a short circuit across a DC capacitor in a DC capacitor bank, such as DC capacitor bank 24. A capacitor may develop a short circuit for a variety of reasons including, for example, high temperatures, mechanical damage, aging, or a power disturbance such as a power surge or a voltage transient. Breakdown of components tests on the ASD circuit, such as those performed to determine compliance of the circuit under Standard UL61800-5-1, may also cause a short circuit across a capacitor. In any case, a short circuit across a capacitor in an ASD circuit will cause the ASD circuit to fail and, if power is allowed to continue to flow to the capacitor bank when one capacitor has a short circuit, an overvoltage condition will exist on the non-shorted capacitor. This overvoltage condition may, in electrolytic capacitors, cause the non-shorted capacitor to catch on fire and possibly explode.
As constructed in the embodiment of FIG. 1, ASD circuit 10 includes a pair of resistors 40 arranged in parallel with capacitors 26, 28 across the DC link 22 in order to mitigate damage to the capacitors 26, 28. However, the resistors 40 are not adequate to fully protect the capacitors, as the resistors cannot prevent power from continuing to flow to a shorted capacitor in DC capacitor bank 24. That is, as diodes 20 of rectifier bridge 18 cannot be controlled to turn off, power can continue to pass through rectifier bridge 18 to DC capacitor bank 24 while a capacitor is shorted. The overvoltage condition on the non-shorted capacitor will cause ASD circuit 10 to be damaged, such that the damaged ASD circuit 10 will not be useable and a user will have to make costly repairs to the ASD circuit or replace the ASD circuit. The user will incur additional economic losses during the downtime period when ASD circuit 10 is not in use.
It would therefore be desirable to provide a system and method for protecting an ASD circuit against fault conditions that could damage the ASD circuit. It would also be desirable for such a system and method to prevent the ASD circuit from catching fire, according to the requirements of Standard UL61800-5-1.